In order to obtain a higher resolution in searches using waves, in general, the length of a sensor has to be lengthened. Assuming that the length of a sensor is "L" and the wavelength is ".lambda.", it is known that resolution ".theta." (.degree.) is expressed by an equation .theta.=50.6 .lambda./L.
In searching a deep sea by using a side-looking scanning sonar or a MNBES which are capable of achieving high resolution, a sensor (transmitting and receiving transducer unit) cannot be made so large, since a low carrier frequency has to be used to decrease propagation loss and the size of the sensor is limited due to installation thereof on ship's bottom. Actually, an upper limit of practical resolution ".theta." is 2.degree.. The resolution of 2.degree. can be said as a sufficiently high resolution for apparatuses of this type. Even with this resolution, when the sea bottom is searched at a depth of 6,000 meters and at a straight line distance of 7,000 meters in a downwardly tilted direction, distance resolution in a right-and-left direction will be as follows: EQU 7,000.times.sin(2.degree.)=244 m
Thus, resolution will be degraded in direct proportion to distance between the sensor and a target. In order to increase resolution on the sea bottom, it may be possible to use a measuring method for bringing down a sensor and maintaining it at a depth close to the sea bottom. In such a case, however, there are faced many difficulties such as the high cost of a towed body and cables and the like, and also problems such as successive measurements at desired points in the water.
There has been introduced a synthetic aperture technique employed in side-looking radar apparatuses (side-watching radars) to achieve high resolution in "Science on Radio Wave Images" (written by Suguru Matsuo and published by Zenkoku Shuppan) or in "Waves and Images" (written by Takuei Sato and others and published by Morikita Shuppan). The technique will be briefly explained by referring to these works.
With radar apparatuses of a general type (radars having an aperture-type antenna), an aircraft Y (FIG. 1) has room to equip only a small-sized antenna so that a beam having a wide range angle .beta. is obtained, as shown in FIG. 1. An ellipse area Q on the ground illuminated thereby will have a resolution which is low as shown in FIG. 1. By applying a synthetic aperture technique to the radar, however, (resolution in the x direction) will be considerably improved as represented by an area A. Distance resolution (z direction) can also be improved by another means (pulse compression) as represented by an area B. As a result, a considerably higher resolution can be obtained as represented by an intersected area C.
Radio waves having a wavelength ".lambda." and a beam angle ".beta." are radiated in a downwardly tilted side direction as shown in FIG. 2. Reception signal levels of echoes resulting therefrom are recorded. A target Tg on the ground is illuminated by radar beams while the aircraft passes points a, b, c, . . . on the flight course. Echoes resulting therefrom are recorded and, at a later time, phase-combined (Echoes are brought in phase and combined), which is equivalent to detecting the target by an antenna substantially having a length ae.
When the target Tg is detected by a radar apparatus at a point C and having a beam angle .beta., the beam is expanded to .beta.R=L at a distance R as explained in the foregoing, which is bearing resolution on the ground. While, with a synthetic aperture antenna, the beam angle .beta.' is expressed as .beta.'=.lambda./ae=.lambda./L. Thus, resolution on the ground will be as .beta.'R=(.lambda./ae) .multidot.R.apprxeq.D. This means that resolution is the same as the length D of the antenna. Further, this resolution is irrespective of distance between a radar apparatus and a target.
With regard to improvement of bearing resolution, there is used the pulse compression technique employed by conventional pulse compression radar apparatuses (chirp radar apparatuses). In order to improve distance resolution, a narrow-width pulse is required to be generated. For this reason, a linearly frequency-modulated pulse is used. There is radiated a transmission signal having its frequency linearly modulated (FM with frequency-shift f) within a transmission pulse width. At a receiver, signals are passed through a matched filter with characteristics having a linear relationship between frequency and delay time so that output signals having an envelope waveform (short pulses) are obtained. The larger the product of frequency shift f by pulse width T, (i.e., the bigger the pulse compression ratio becomes, the narrow the pulse width results).
Accordingly, if the synthetic aperture technique is applied to underwater detection systems, it can be easily expected that improvement of resolution is obtained. As will be explained hereinafter, this has not been yet achieved, since propagation speed of ultrasonic signals in the water is considerably slower than that of radio waves and this is the principal cause for the synthetic aperture technique not being more widely applied.